By utilizing their high specific strength and specific elasticity, fiber-reinforced composite materials in which carbon, glass, aramid or other structural fiber is used as reinforcing fiber have been used, in recent years, in structural materials for boats, automobiles, and the like, in sports equipment such as tennis rackets, golf clubs, and fishing rods, and in common industrial applications.
Methods for producing fiber-reinforced composite materials include methods in which an uncured matrix resin is infused into reinforcing fiber to form a sheet-form prepreg intermediate, followed by curing, and resin transfer molding methods in which liquid-form resin is made to flow into reinforcing fiber that has been placed in a mold to produce an intermediate, followed by curing. Of these production methods, with those methods that employ prepregs, the fiber-reinforced composite material is normally obtained by hot-pressing subsequent to layering multiple sheets of prepreg. The matrix resin that is used in the prepreg is commonly a thermosetting resin from the standpoint of productivity considerations.
Phenol resins, melamine resins, bismaleimide resins, unsaturated polyester resins, epoxy resins, and the like have been used as the thermosetting resin. However, from the standpoint of improving moisture resistance and heat resistance, investigations have been progressing in recent years concerning the use of benzoxazine resins as matrix resins in fiber-reinforced composite materials (e.g., see WO 03/018674).
However, benzoxazine resin is essentially a brittle material with poor toughness and elongation, and, when used alone, curing the resin requires a lengthy time of 3 h at a high temperature of 200° C. Moreover, due to its high viscosity, benzoxazine resin has the disadvantage of poor tackiness and draping properties when used as a prepreg in combination with reinforcing fiber.
In order to improve the curing properties of benzoxazine resin, the combination of benzoxazine resin, a Novolak compound, and a cationic polymerization initiator has been disclosed (e.g., see JP-2002-128987-A). However, this publication does not describe use as a fiber-reinforced composite material. In addition, although the disclosed compositions have sufficiently high reactivity, reactions progress even at room temperature due to inferior latency, and there have thus been problems with rapid degradation when used in prepregs and with poor tackiness and draping properties.
In addition, it has been disclosed that high tensile strength utilization is obtained with fiber-reinforced composite materials, provided that the tensile break elongation and fracture toughness KIc of the matrix resin satisfies a specific relationship (e.g., see JP-09-235397-A). With benzoxazine resins as well, investigations have been carried out concerning blending toughening agents such as thermoplastic resins and thermoplastic elastomers, but the solubility of these toughening agents in benzoxazine resins is low, and there have been problems with blending amounts that are sufficient for improving tensile strength. In addition, blending toughening agents increases viscosity, and there have been problems with processing and handling during prepreg production, as well as inferior tackiness and draping properties when used in prepregs.
Although multifunctional phenols have been used as curing agents along with epoxy resins in the prior art (e.g., see JP-2012-36347-A), there have been problems with deficient tackiness and draping properties when used as a prepreg due to a resulting increase in viscosity of the benzoxazine resin composition.